ReCrNi alloy coating for diffusion barrier

ABSTRACT

Disclosed is a ReCrNi alloy coating for diffusion barrier, such as a high-temperature equipment member, consisting essentially of a ternary alloy and having, except for inevitable impurities, an atomic composition of 20% to 80% Re, an atomic composition of 20% to 60% Cr, and an atomic composition of 5% to 40% Ni. Even if the alloy coating for diffusion barrier includes a diffusion layer containing at least one of the group consisting of Al, Si and Cr, a desired alloy composition of the alloy coating for diffusion barrier can be assured by a surface coating process and diffused components from the substrate while substantially preventing the diffusion of the elements of the diffusion layer during a homogenizing heat treatment. The alloy coating for diffusion barrier may include a Re-containing-alloy stress relief layer inserted between the film and the substrate. The ReCrNi alloy coating can suppress the deterioration of the substrate and the coating layer due to the reaction therebetween to provide an extended life span of the equipment member.

TECHNICAL FIELD

The present invention relates to a technology for extending the life span of a member for use in high-temperature equipment, such as a gas turbine blade, a turbine blade for a jet engine or a boiler tube.

BACKGROUND ART

(1) Thermal Barrier Coating (TBC)

A high-temperature equipment member, such as an industrial gas turbine blade or a boiler tube, has a coated surface to obtain enhanced heat resistance and corrosion resistance, in many cases. With regard to heat resistance, a ceramics coating, so-called thermal barrier coating (TBC), is typically applied to the equipment member.

In use of ceramics, an alloy layer, so-called undercoat (or bondcoat), is generally inserted between a ceramics layer and a metal substrate to assure adhesion therebetween, because the relatively large difference in thermal expansion coefficient between ceramics and substrate metal is likely to cause peeling of the ceramics layer at the TBC/substrate interface. However, under an ultra-high temperature environment of about 800 to 1200° C., the undercoat will be deteriorated due to reaction with the substrate, or the peeling of the ceramics layer will occur due to an oxide layer thickly grown on the surface of the undercoat. This leads to a serious problem that the equipment member has a short life span of several months.

(2) Al (or Cr, Si) Diffusion Coating Process

With regard to corrosion resistance, the equipment member is typically subjected to an Al (or Cr, Si) diffusion coating process. However, under an ultra-high temperature environment of about 800 to 1200° C., a protective film cannot be stably maintained for a long time-period due to significantly high diffusion rate and high reactivity of a metal element contributing to corrosion resistance. Further, under a severe corrosive environment containing Cl or S, an metal element, such as Cr or Al, constituting the protective film will be quickly consumed even in a temperature range of 500 to 800° C., which leads to difficulties in maintaining the protective film stably for a long time-period, and a serious problem of significantly shortened life span in the equipment member.

(3) Ni—Cr Thermal Spraying

For providing enhanced corrosion resistance, a high Ni-high Cr alloy thermal sprayed coating is applied to the equipment member, in some cases. However, this case involves the same problem as that in the above (2).

(4) TBC System Combined with Re-added Undercoat

Japanese Patent Laid-Open Publication No. 11-61439 discloses a TBC system including an undercoat with Re (Rhenium) added thereto at 12 weight % or more (several % by atomic composition). Japanese Patent Laid-Open Publication No. 2000-511236 titled “Structural Component including Superalloy Substrate and Layer Structure formed thereon, and Production Method thereof” discloses an undercoat for a TBC, containing Re in the range of 35 to 60 weight % (about 15 to 30% by atomic composition). These publications discloses neither the function of Re nor the effect of the Re addition. U.S. Pat. No. 6,299,986 discloses a superalloy article in which a barrier coating containing 4 wt % or less of Re is formed on a Ni-based superalloy substrate containing 5.0 to 7.0 wt % of Re.

(5) Re Coating & Re Alloy Coating

Japanese Patent Laid-Open Publication No. 03-120327 discloses a protective film containing 1 to 20 wt % of Re and 22 to 50 wt % of Cr. Japanese Patent Laid-Open Publication No. 09-143665 discloses a production method of a high-temperature member comprising pure Re, Re—Mo alloy or Re—W alloy. The method disclosed in this publication is intended to produce a Re or Re alloy structural member which is used independently in the form of a thin plate.

DISCLOSURE OF INVENTION

Under existing circumstances involving the aforementioned unsolved problems, there is no other choice but to lower the operating temperature of the high-temperature equipment at the sacrifice of the performance thereof so as to assure a desired life span of the equipment member.

Despite of various researches on a method using Re or Re alloy as a diffusion barrier, the inventors could not find out an intended layer structure capable of stably providing a diffusion barrier performance.

[1. Shyouji, Hisamatsu, Hayashi, Narita, “Development guideline of technologies for improvement of oxidation resistance as countermeasures for ultra-high temperature—Application of rhenium-based alloy coating to Ni-based superalloy—”, the 123rd Committee Report, Heat-Resistant Material, Japan Society for the Promotion of Science, Vol. 141, pp 127 (March 2000);

2. T. Narita et al., “Rhenium coating as a diffusion barrier on a nickel-based superalloy in high temperature oxidation”, Proc of HTCP2000. pp 351, Science Reviews, Hokkaido (Sep. 2000);

3. Yoshida, Hisamatsu, Hayashi, Noguchi, Yakuwa, Miyasaka, “Formation of diffusion barrier layer through plating process & Oxidation resistance”, Journal of the 47th “Materials & Environments” symposium, pp 141, Japan Society of Corrosion Engineering, Yamaguchi (October 2000);

4. Hisamatsu, Yoshida, Hayashi, Narita, Murakami, Harada, “Diffusion barrier characteristics of Re coating film & Oxidation resistance of Ni-based superalloy”, Journal of the 47th “Materials & Environments” symposium, pp 153, Japan Society of Corrosion Engineering, Yamaguchi (October 2000); and

5. T. Narita et al., “Application of rhenium coating as a diffusion barrier to improve the high temperature oxidation resistance of nickel-based superalloy”, Paper No. 01157, CORROSION 2001, NACE Intl, Huston (March 2001)].

The present invention is directed to suppress the problem of the anti-corrosion coating formed through the conventional TBC system, Al (or Cr, Si) diffusion coating process or thermal splaying, or the deterioration of a substrate and a coating layer due to the reaction therebetween, so as to provide intended life span of a high-temperature equipment member.

A Re (or Ir, Rh, Pt, W) alloy layer can be coated on the surface of a substrate, or inserted between a substrate and a TBC layer to provide an excellent diffusion barrier layer. However, if Re in the Re alloy coating is contained at a low concentration, or a specific alloy, such as Re—Ni binary alloy, incapable of forming a stable alloy phase with Re under high temperature is used, a Re—Ni alloy phase will be separated into a Re-rich phase and a counter-element (e.g. Ni)-rich phase, resulting in deteriorated diffusion barrier function.

The inventors have found that, among Re alloy coatings formed on a substrate, a ReCrNi alloy coating particularly has a high diffusion barrier function and a wide range of the σ phase serving as a diffusion barrier layer, and exhibits a sufficient diffusion barrier performance even at a relatively low concentrate of Re.

That is, a pure Re alloy or a Re alloy has the risk of cracks to be introduced therein in connection with the use under heat cycle because it essentially has a thermal expansion coefficient significantly different from that of a Ni-based (or Fe-based, Co-based) alloy used in a substrate. In terms of this problem, the diffusion barrier layer in the present invention is arranged to have a relatively low Re concentration to assure sufficient adhesion with the substrate.

The present invention provides an excellent diffusion barrier layer in consideration of actual operating environments based on the above knowledge.

Specifically, the present invention provides a ReCrNi alloy coating for diffusion barrier, consisting essentially of a ternary alloy and having, except for inevitable impurities, an atomic composition of 20% to 75% Re, an atomic composition of 20% to 60% Cr, and an atomic composition of 5% to 40% Ni. The ReCrNi alloy coating allows the substrate to maintain an excellent heat resistance and corrosion resistance for a long time-period.

The alloy coating for diffusion barrier may include a stress relief layer inserted between the substrate and the alloy coating. This structure can suppress a crack of the alloy coating caused by the difference in thermal expansion between the substrate and alloy coating and the substrate to maintain the alloy coating as a continuous layer, so as to more effectively utilize the heat resistance and corrosion resistance of the alloy coating.

The alloy coating for diffusion barrier may further include a diffusion layer laminated on the ReCrNi alloy coating. The diffusion layer contains at least one selected from the group consisting of Al, Si and Cr. In this structure, the ReCrNi alloy coating can prevent the diffusion of an element causing the deterioration of corrosion resistance (e.g. Ti, Nb, Ta) from the substrate to the diffusion layer, and the diffusion of an element causing the deterioration of phase stability (e.g. Al, Si, Cr) from the diffusion layer to the substrate. Thus, an excellent oxidation resistance and the strength of the substrate can be maintained for a long time-period.

The alloy coating for diffusion barrier may further include a heat barrier ceramics laminated on the ReCrNi alloy coating. The heat barrier ceramics allows the above materials to be used under high temperature.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a phase diagram of a ReCrNi alloy under high temperature, which comparatively shows an alloy composition of a diffusion barrier ReCrNi alloy coating of the present invention and a conventional example.

FIG. 2 is a schematic diagram showing the sectional structure of a Ni-based alloy in Inventive Example 1.

FIG. 3 is a schematic diagram showing the sectional structure of a Ni-based alloy in Inventive Example 2.

FIGS. 4( a) to 4(d) are schematic diagrams showing the sectional structures of Ni-based alloys in Comparative Examples 1 to 4, respectively.

FIG. 5 is a schematic diagram showing the sectional structure of the Ni-based alloy in Inventive Example 1 after oxidized in the atmosphere at 1100° C. for one month.

FIG. 6 is a schematic diagram showing the sectional structure of the Ni-based alloy in Inventive Example 2 after oxidized in the atmosphere at 1100° C. for one month.

FIGS. 7( a) to 7(d) are schematic diagrams showing the sectional structures of Ni-based alloys in Comparative Examples 1 to 4 after oxidized in the atmosphere at 1100° C. for one month, respectively.

FIG. 8 is a graph showing the thickness of an oxidized scale of each of Inventive Examples 1 and 2 and Comparative Examples 1 to 4 after oxidized in the atmosphere at 1100° C. for one month.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention will now be described in detail.

The present invention relates to a ReCrNi alloy coating for diffusion barrier, consisting essentially of a ternary alloy. The Re alloy coating, except for inevitable impurities, has an atomic composition of 20% to 75% Re, an atomic composition of 20% to 60% Cr, and an atomic composition of 5% to 40% Ni. In order to sufficiently bring out the function of the alloy coating as a diffusion barrier layer, Re is required to be alloyed with Cr and Ni. Further, an atomic composition of the alloy coating is required to have 20% to 75% of Re, 20% to 60% of Cr, and 5% to 40% of Ni.

If Re is less than 20%, the rate of (the element other than Re)-rich phase will be increased to cause the deterioration in diffusion barrier function of the alloy coating. If Re is 80% or more, the σ phase serving as the diffusion barrier layer will become thermodynamically unstable, and separated into a Re single-phase and a Ni-rich phase. Differently from a Re alloy phase, the Re single-phase is apt to allow cracks to be introduced therein due to thermal shock. Further, due to poor adhesion between the Re single-phase and the substrate, the alloy coating cannot stably exist as a diffusion layer on the surface of the substrate for a long time-period. In addition, the Re alloy layer separated into two phases has a poor diffusion barrier function due to the separated Ni-rich phase allowing elements to be actively diffused therethrough. Therefore, the concentration of Re is limited in the range of 20% to 75% by atomic composition. Preferably, the concentration of Re is in the range of 35% to 75% by atomic composition.

Cr can be alloyed with Re to form a stable phase so-called σ phase. This σ phase allows the alloy coating to bring out an excellent diffusion barrier function. If Cr is less than 20%, a Re single-phase will be created, or no π phase will be formed. If Cr is greater than 60%, a Cr-rich phase will be created in larger rate than that of the σ phase to cause the deterioration in diffusion barrier function of the alloy coating. Thus, Cr is limited in the range of 20% to 60%. Preferably, the concentration of Cr is in the range of 25% to 50% by atomic composition.

Ni can be alloyed with a Re—Cr alloy to effectively extend the stable region of the σ phase. A stable σ phase in the Re—Cr alloy can be obtained at a Re concentration of about 50 to 70 atomic %. By contrast, if it is further alloyed with Ni, the stable region of the σ phase will be extended in the range of about 20 to 80 atomic % Re.

FIG. 1 shows a phase diagram of a calculated phase diagram of a ReCrNi alloy at 1150° C. According to the latest inventor's research, it has been verified that the stable region of the σ phase is actually wider on the side of the high Ni and the high Re.

Based on the above description, an alloy composition range effective to a diffusion barrier layer is shown in FIG. 1 together with the calculated phase diagram of the ReCrNi alloy. For comparison, respective alloy composition ranges of conventional examples are also shown in FIG. 1. As shown in FIG. 1, the composition ranges of the alloy coatings disclosed in the Japanese Patent Laid-Open Publication Nos. 2000-511236 and No. 11-61439 as the conventional examples are a Cr-rich phase and a Ni-rich phase, respectively. Thus, these alloy coating cannot act as a diffusion barrier layer.

The film disclosed in the Japanese Patent Laid-Open Publication No. 09-143667 contains none of Ni and Cr. Thus, the film is apt to be cracked due to thermal shock, and poor in adhesion with a substrate. As described in the specification of the Japanese Patent Laid-Open Publication No. 09-143667, this film is used by itself without being coated on a substrate. If this film is used after coated on a substrate, the film will be readily cracked to cause the deterioration in heat resistance and corrosion resistance.

By contrast, as seen in FIG. 1, the alloy coating of the present invention has a composition centered on an σ phase region serving as an excellent diffusion barrier layer so as to bring out an excellent diffusion barrier function. In addition, the alloy coating consisting essentially of a ReCrNi ternary alloy can contain a larger amount of Ni which is a primary component of the heat-resistant alloy used in the substrate. Thus, the alloy coating has excellent adhesion with the substrate, so that the alloy coating coated on the substrate can be used while adequately maintaining the coated state for a long time-period.

While the ReCrNi alloy coating can be preferably formed through a magnetron sputtering process, a physical deposition process, a chemical deposition process or thermal spraying process may be used to form the same alloy coating. A process for forming the Re alloy coating with the desired composition is not limited to these processes, but the desired Re alloy coating may be formed by diffusing alloy components of the substrate to the film through a heat treatment. In either case, the alloy coating is preferably subjected to a heat treatment at a high temperature in vacuum under a non-oxidation atmosphere, such as an inert atmosphere, to homogenize the composition and structure of the Re layer or Re alloy layer coated on the substrate. In this manner, when the alloy coating for diffusion barrier of the present invention includes a diffusion layer containing at least one selected from the group consisting of Al, Si and Cr, the components of the substrate can be diffused while substantially preventing the elements of Al, Si and Cr from being diffused.

When the alloy coating for diffusion barrier of the present invention includes a stress relief layer inserted between the substrate and the alloy layer, the stress relief layer is preferably a ReCrNi alloy layer, for example, wherein the Re concentration is reduced by about 5 to 20 atomic %, and the concentration of Ni is increased by about 5 to 20 atomic %, as compared to those of the diffusion barrier layer. This structure can suppress cracks in the alloy coating due to the difference in thermal expansion coefficient between the substrate and the alloy coating so as to maintain the alloy coating as a continuous layer to more effectively bring out the heat resistance and corrosion resistance of the alloy coating.

As a process for laminating the diffusion layer containing at least one selected from the group consisting of Al, Si and Cr, a conventional process, such as a pack process or a CVD process, may be appropriately used. For example, a receptor layer for Al, Si or Cr to be diffused may be formed by plating a metal layer containing at least one selected from the group consisting of Ni, Fe and Co, on a ReCrNi alloy coating, and subjecting the alloy coating with the metal layer to a diffusion treatment at a high temperature to form an alloy layer containing these elements.

When the alloy coating for diffusion barrier of the present invention further includes a heat barrier ceramics laminated on the diffusion-barrier alloy coating, the ceramics preferably includes at least one selected from the group consisting of ZrO₂, CaO, MgO, SiO₂ and Al₂O₃. The heat barrier ceramics can provide a lower temperature of the internal alloy layer to suppress the growth of oxide on the surface of the alloy coating and the diffusion between the alloy coating and the substrate so as to maintain the structural stability of the alloy coating and the substrate for an extended time-period.

EXAMPLE Inventive Example 1

After an 45 atomic % Re-55 atomic % Cr alloy coating was coated on a Ni-based alloy of Inconel 738LC (Ni-16% Cr-8.5% Co-0.9% Nb-1.7% Mo-2.6% W-3.4 Ti-3.4% Al (weight %)) for use in a rotor blade or stationary blade of a gas turbine, the Ni-based alloy was Ni-plated, and then subjected to an Al diffusion coating. Specifically, the alloy coating was formed by coating a 45 atomic % Re-55 atomic % Cr alloy on the surface of the substrate through a magnetron sputtering process using the Re—Cr alloy as a target, and then subjecting the substrate with the alloy layer to a homogenizing heat treatment in vacuum at 1100° C. for 5 hours.

A Watts bath was used for the Ni plating. Specifically, the plating conditions were set as follows: a bath composition [the weight ratio of NiSO₄.7H₂O:NiCl₂: H₃BO₃=20:3:2], pH=5, a bath temperature of 50° C., and a current density of 50 mA/cm². The Al diffusion coating was performed under the condition that the substrate with coated Re alloy and plated-Ni was heated at 1000° C. in vacuum for 5 hours while being immersed in a mixed powder of a Ni-50 atomic % Al alloy powder and an Al₂O₃ powder.

FIG. 2 shows the sectional structure of the Ni-based alloy after the treatment. Further, Table 1 shows the analysis result of the composition of each point in FIG. 2 using an electron probe microanalyzer (EPMA). The data (1) to (5) in Table 1 correspond to the points (1) to (5) in FIG. 2, respectively.

TABLE 1 (atomic %) (1) (2) (3) (4) (5) Re 0.0 0.2 38.1 0.0 0.0 Ni 60.1 53.0 9.2 46.5 48.3 Cr 18.8 26.9 48.2 2.8 0.9 Co 7.7 7.3 1.9 1.2 0.7 W 1.0 0.6 1.0 0.0 0.0 Mo 0.8 0.7 0.9 0.0 0.0 Ta 0.4 0.4 0.1 0.0 0.0 Nb 0.5 0.5 0.1 0.0 0.0 Al 6.8 6.9 0.3 49.5 50.0 Ti 3.9 3.5 0.2 0.0 0.0

As seen in Table 1, after the post-treatment, the 45 atomic % Re-55 atomic % Cr alloy coating is formed as an alloy coating having an atomic composition of 38.1% Re, 48.2% Cr and 9.2% Ni. That is, the alloy coating has a composition of the σ phase (see FIG. 1). The other alloy element other than Re, Cr and Ni is less than 2% by atomic component, and has no significant function as a diffusion barrier.

Inventive Example 2

After an 40 atomic % Re-50 atomic % Cr-10 atomic % Ni alloy coating was coated on a Ni-based alloy of Inconel 738LC (Ni-16% Cr-8.5% Co-0.9% Nb-1.7% Mo-2.6% W-3.4 Ti-3.4% Al (weight %)) for use in a rotor blade or stationary blade of a gas turbine, the Ni-based alloy was Ni-plated, and then subjected to an Al diffusion coating. Specifically, the alloy coating was formed by coating a 40 atomic % Re-50 atomic % Cr-10 atomic % Ni alloy on the surface of the substrate through a magnetron sputtering process using the Re—Cr alloy as a target, and then subjecting the substrate with the alloy layer to a homogenizing heat treatment in vacuum at 1100° C. for 5 hours.

A Watts bath was used for the Ni plating. Specifically, the plating conditions were set as follows: a bath composition [the weight ratio of NiSO₄.7H₂O:NiCl₂:H₃BO₃=20:3:2], pH=5, a bath temperature of 50° C., and a current density of 50 mA/cm². The Al diffusion coating was performed under the condition that the substrate with coated Re alloy and plated-Ni was heated at 1000° C. in vacuum for 5 hours while being immersed in a mixed powder of a Ni-50 atomic % Al alloy powder and an Al₂O₃ powder.

FIG. 3 shows the sectional structure of the Ni-based alloy after the treatment. Further, Table 2 shows the analysis result of the composition of each point in FIG. 3 using an electron probe microanalyzer (EPMA). The data (1) to (5) in Table 1 correspond to the points (1) to (5) in FIG. 3, respectively.

TABLE 2 (atomic %) (1) (2) (3) (4) (5) Re 0.0 0.3 24.2 0.0 0.0 Ni 59.8 53.0 31.8 45.4 46.9 Cr 17.9 26.9 39.5 4.9 2.1 Co 8.0 7.2 1.9 0.8 0.8 W 1.0 0.6 0.9 0.0 0.0 Mo 1.0 0.7 1.1 0.0 0.0 Ta 0.5 0.4 0.2 0.0 0.0 Nb 0.5 0.5 0.1 0.0 0.0 Al 7.2 6.9 0.3 48.9 50.2 Ti 4.1 3.5 0.0 0.0 0.0

As seen in Table 2, after the post-treatment, the 40 atomic % Re-50 atomic % Cr-10 atomic Ni alloy coating is formed as an alloy coating having an atomic composition of 24.2% Re, 39.5% Cr, and 31.8% Ni. This composition corresponds to a point slightly sifted from the stable region of the σ phase toward high Ni side in the calculated phase diagram of FIG. 1. As with the Inventive Example 1, the other alloy element other than Re, Cr and Ni is less than 2% by atomic component, and has no significant function as a diffusion barrier.

Comparative Examples 1 to 4

FIGS. 4( a), 4(b), 4(c) and 4(d) show the sectional structures of a pure Ni-based alloy (Inconel 738LC) as Comparative Example 1, a Ni-based alloy (Inconel 738LC) subjected only to an Al diffusion coating as Comparative Example 2, a Ni-based alloy (Inconel 718) subjected to a Ni-plating and an Al diffusion coating after coating a 20 atomic % Re-60 atomic % Ni-20 atomic % Cr alloy coating thereon, as Comparative Example 3, and a Ni-based alloy (Inconel 738LC) subjected to a Ni-plating and an Al diffusion coating after coating a 20 atomic % Re-80 atomic % Cr alloy coating thereon, as Comparative Example 4, respectively.

The Re alloy coating was formed by coating a 20 atomic % Re-60 atomic % Ni-20 atomic % Cr alloy in Comparative Example 3, or a 20 atomic % Re-80 atomic % Cr alloy in Comparative Example 4, on the substrate through a magnetron sputtering process, and subjecting the substrate with the alloy to a heat treatment at 1100° C. in vacuum for 5 hours. The respective conditions of the Ni plating and the Al diffusion coating process were the same as those in Inventive example 1.

Table 3 shows the analysis result of the composition of each point of the section of the Ni-based alloy in Comparative Example 1 according to EPMA. The data (1) to (3) in Table 3 correspond to the points (1) to (3) in FIG. 4( a), respectively.

TABLE 3 (atomic %) (1) (2) (3) Re 0.0 0.0 0.0 Ni 59.8 60.6 60.0 Cr 17.6 17.8 17.9 Co 8.4 8.3 8.4 W 1.0 1.0 0.9 Mo 1.2 1.1 1.1 Ta 0.6 0.5 0.5 Nb 0.5 0.5 0.4 Al 6.9 7.0 7.0 Ti 4.0 3.8 3.8

Table 4 shows the analysis result of the composition of each point of the section of the Ni-based alloy in Comparative Example 2 according to EPMA. The data (1) to (5) in Table 4 correspond to the points (1) to (5) in FIG. 4( b), respectively.

TABLE 4 (atomic %) (1) (2) (3) (4) (5) Re 0.0 0.0 0.0 0.0 0.0 Ni 59.8 56.4 53.9 39.3 42.0 Cr 17.6 15.2 12.0 6.1 4.0 Co 8.4 7.1 4.3 2.8 2.7 W 1.1 0.8 0.3 0.1 0.0 Mo 1.1 0.9 0.5 0.0 0.0 Ta 0.6 0.5 0.4 0.2 0.1 Nb 0.5 0.4 0.3 0.1 0.1 Al 6.9 15.2 25.1 49.5 49.9 Ti 4.0 3.5 3.2 1.9 1.2

Table 5 shows the analysis result of the composition of each point of the section of the Ni-based alloy in Comparative Example 3 according to EPMA. The data (1) to (5) in Table 5 correspond to the points (1) to (5) in FIG. 4( c), respectively.

TABLE 5 (atomic %) (1) (2) (3) (4) (5) Re 0.0 1.0 9.2 0.9 0.0 Ni 59.6 57.7 47.9 44.0 43.0 Cr 18.0 15.2 19.4 10.5 6.0 Co 8.4 7.1 4.3 3.2 2.2 W 0.9 0.8 0.3 0.1 0.0 Mo 1.1 0.9 0.6 0.3 0.0 Ta 0.5 0.5 0.2 0.2 0.1 Nb 0.5 0.4 0.2 1.0 0.0 Al 7.0 12.9 16.5 39.5 47.8 Ti 4.0 3.5 1.4 1.2 0.9 O 0.0 0.0 0.0 0.0 0.0

Table 6 shows the analysis result of the composition of each point of the section of the Ni-based alloy in Comparative Example 4. The data (1) to (5) in Table 6 correspond to the points (1) to (5) in FIG. 4( d), respectively.

TABLE 6 (atomic %) (1) (2) (3) (4) (5) Re 0.0 2.1 8.9 1.3 0.0 Ni 59.4 51.3 7.9 44.1 46.9 Cr 18.1 22.8 56.2 12.3 6.0 Co 8.2 7.1 3.9 2.7 2.2 W 0.8 0.7 0.5 0.2 0.0 Mo 1.2 0.9 0.9 0.3 0.0 Ta 0.5 0.4 0.2 0.1 0.0 Nb 0.6 0.4 0.3 0.2 0.1 Al 7.2 11.3 19.2 37.8 43.9 Ti 4.0 3.0 2.0 1.0 0.9

In each of Comparative Examples 2 to 4, the Al concentration in the vicinity of the surface of the coating layer composed of the Re alloy coating and the Ni—Al alloy diffusion layer is about 40 atomic %, which is a sufficient amount of Al to form Al₂O₃ serving as a dense protective layer. However, differently from Inventive Examples 1 and 2, in all of Comparative examples 2 to 4, Al and Ti/Nb are clearly diffused to the substrate and the coating layer, respectively.

Further, in Comparative Example 3, after the post-treatment, the 20 atomic % Re-60 atomic % Ni-20 atomic % Cr alloy coating is formed as an alloy coating having an atomic composition of 9.2% Re, 47.9% Ni, 19.4% Cr, 4.3% Co, 16.5% Al and 1.4% Ti. Leaving Al and Ti out of consideration, the composition corresponds to the Ni-rich region in FIG. 1, which is close to the composition of the alloy disclosed in the Japanese Patent Laid-Open Publication No. 11-61439 as one of the conventional examples.

In Comparative Example 4, after the post-treatment, the 20 atomic % Re-80 atomic % Cr alloy coating is formed as an alloy coating having an atomic composition of 8.9% Re, 7.9% Ni, 56.2% Cr, 3.9% Co, 19.2% Al and 2.0% Ti. Leaving Al and Ti out of consideration, the composition corresponds to the Cr-rich phase in FIG. 1, which is close to the composition of the alloy disclosed in the Japanese Patent Laid-Open Publication No. 2000-511236 as one of the conventional examples.

FIGS. 5 and 6, and FIGS. 7( a) to 7(d) show the sectional structures of Inventive Examples 1 and 2, and Comparative Examples 1 to 4 after oxidized in the atmosphere at 1100° C. for one month. FIGS. 2 and 3 correspond to FIGS. 5 and 6, respectively, and FIGS. 4( a) to 4(d) correspond to FIGS. 7( a) to 7(d), respectively.

Table 7 shows the analysis result of the composition of each point of the section of an oxidized test piece of the Ni-based alloy in Inventive Example 1. The data (1) to (6) in Table 7 correspond to the points (1) to (6) in FIG. 5, respectively.

TABLE 7 (atomic %) (1) (2) (3) (4) (5) (6) Re 0.0 0.0 52.5 0.0 0.0 0.0 Ni 60.1 58.0 1.8 48.0 50.2 0.0 Cr 18.4 21.5 42.1 1.9 0.4 0.0 Co 8.0 7.7 1.3 0.3 0.1 0.0 W 1.0 0.7 1.0 0.0 0.0 0.0 Mo 0.8 0.6 0.7 0.0 0.0 0.0 Ta 0.4 0.4 0.2 0.0 0.0 0.0 Nb 0.4 0.5 0.1 0.0 0.0 0.0 Al 7.0 7.0 0.3 49.8 49.3 39.8 Ti 3.9 3.6 0.0 0.0 0.0 0.0 O 0.0 0.0 0.0 0.0 0.0 60.2

Table 8 shows the analysis result of the composition of each point of the section of an oxidized test piece of the Ni-based alloy in Inventive Example 2. The data (1) to (6) in Table 8 correspond to the points (1) to (6) in FIG. 6, respectively.

TABLE 8 (atomic %) (1) (2) (3) (4) (5) (6) Re 0.0 0.2 23.9 0.0 0.0 0.0 Ni 59.7 53.3 31.3 45.5 47.4 0.0 Cr 18.1 25.5 40.3 4.5 2.3 0.0 Co 8.0 7.8 1.6 1.2 0.8 0.0 W 1.0 0.7 1.2 0.0 0.0 0.0 Mo 1.0 0.8 1.2 0.0 0.0 0.0 Ta 0.5 0.4 0.2 0.0 0.0 0.0 Nb 0.5 0.5 0.1 0.0 0.0 0.0 Al 7.2 7.1 0.2 48.8 49.5 40.2 Ti 4.0 3.7 0.0 0.0 0.0 0.0 O 0.0 0.0 0.0 0.0 0.0 59.8

Table 9 shows the analysis result of the composition of each point of the section of an oxidized test piece of the Ni-based alloy in Comparative Example 1. The data (1) to (6) in Table 9 correspond to the points (1) to (6) in FIG. 7( a), respectively.

TABLE 9 (atomic %) (1) (2) (3) (4) (5) (6) Re 0.0 0.0 0.0 0.0 0.0 0.0 Ni 59.7 61.9 62.1 5.2 24.3 32.3 Cr 18.1 17.1 16.9 20.9 13.1 7.9 Co 8.0 8.9 9.9 1.2 4.9 9.2 W 1.0 1.2 1.0 0.0 0.0 0.0 Mo 1.0 1.3 1.2 0.0 0.0 0.0 Ta 0.5 0.3 0.3 0.0 0.2 0.2 Nb 0.5 0.5 0.4 0.0 0.1 0.4 Al 7.2 6.3 6.1 14.9 6.7 4.4 Ti 4.0 2.5 2.1 0.0 2.1 2.9 O 0.0 0.0 0.0 57.8 48.6 42.7

Table 10 shows the analysis result of the composition of each point of the section of an oxidized test piece of the Ni-based alloy in Comparative Example 2. The data (1) to (6) in Table 10 correspond to the points (1) to (6) in FIG. 7( b), respectively.

TABLE 10 (atomic %) (1) (2) (3) (4) (5) (6) Re 0.0 0.0 0.0 0.0 0.0 0.0 Ni 59.6 56.4 58.9 61.8 64.2 12.9 Cr 17.9 16.4 13.2 8.0 5.5 3.8 Co 8.0 8.5 6.6 5.1 3.7 7.2 Co 8.0 8.5 6.6 5.1 3.7 7.2 W 1.0 1.2 1.0 0.3 0.1 0.1 Mo 1.0 1.3 1.2 0.0 0.0 0.0 Ta 0.5 0.3 0.3 0.1 0.2 0.1 Nb 0.6 0.5 0.4 0.2 0.1 0.1 Al 7.3 12.9 16.3 23.9 25.7 23.6 Ti 4.1 2.5 2.1 0.6 0.5 0.7 O 0.0 0.0 0.0 0.0 0.0 51.5

Table 11 shows the analysis result of the composition of each point of the section of an oxidized test piece of the Ni-based alloy in Comparative Example 3. The data (1) to (6) in Table 11 correspond to the points (1) to (6) in FIG. 7( c), respectively.

TABLE 11 (atomic %) (1) (2) (3) (4) (5) (6) Re 0.0 1.5 8.4 1.6 0.0 0.0 Ni 59.6 55.3 43.0 58.0 62.7 3.8 Cr 18.0 15.2 19.0 10.5 6.0 0.9 Co 8.4 7.1 5.9 4.1 3.3 4.5 W 0.9 0.8 0.5 0.1 0.0 0.1 Mo 1.1 0.9 0.6 0.3 0.0 0.0 Ta 0.5 0.5 0.4 0.2 0.1 0.1 Nb 0.5 0.4 0.3 0.1 0.0 0.1 Al 7.0 15.2 20.5 24.1 27.1 30.2 Ti 4.0 3.1 1.4 1.0 0.8 0.5 O 0.0 0.0 0.0 0.0 0.0 59.8

Table 12 shows the analysis result of the composition of each point of the section of an oxidized test piece of the Ni-based alloy in Comparative Example 4. The data (1) to (6) in Table 12 correspond to the points (1) to (6) in FIG. 7( d), respectively. Further, FIG. 8 shows the thickness of each oxidized scale created these surfaces.

TABLE 12 (atomic %) (1) (2) (3) (4) (5) (6) Re 0.0 2.1 8.6 2.0 0.0 0.0 Ni 59.4 42.7 11.6 51.9 59.2 4.1 Cr 18.1 26.9 50.7 16.7 11.2 0.9 Co 8.2 7.1 4.5 3.1 1.9 3.9 W 0.8 0.7 0.5 0.3 0.1 0.2 Mo 1.2 0.9 0.9 0.4 0.0 0.0 Ta 0.5 0.4 0.2 0.1 0.0 0.1 Nb 0.6 0.4 0.3 0.2 0.1 0.1 Al 7.2 15.8 20.7 24.1 26.8 33.0 Ti 4.0 3.0 2.0 1.2 0.7 0.5 O 0.0 0.0 0.0 0.0 0.0 57.2

In Inventive Example 1, a dense oxidized scale with a thin thickness of about 2 μm containing Al₂O₃ almost without other elements was formed on the surface of the coating layer, and no peeling of the film was observed. In addition, neither the diffusion of the elements (e.g. Ti and Nb) of the substrate to the Ni—Al alloy diffusion layer nor the diffusion of Al of the Ni—Al alloy diffusion layer to the substrate were practically observed. The concentration Al in the Ni—Al alloy diffusion layer was maintained at about 50% approximately equal to that before oxidation, and no significant change was observed in the composition of the substrate. The Re alloy diffusion barrier layer also exhibited substantially the same composition as that before oxidation.

In Inventive Example 2, a dense oxidized scale with a thin thickness of about 3 μm containing Al₂O₃ almost without other elements was formed on the surface of the coating layer, and no peeling of the film was observed. As with the Inventive Example 1, neither the diffusion of the elements (e.g. Ti and Nb) of the substrate to the Ni—Al alloy diffusion layer nor the diffusion of Al of the Ni—Al alloy diffusion layer to the substrate were practically observed. The concentration Al in the Ni—Al alloy diffusion layer was maintained at about 50% approximately equal to that before oxidation, and no significant change was observed in the composition of the substrate. The Re alloy diffusion barrier layer also exhibited substantially the same composition as that before oxidation.

In contrast, the following results were obtained from test peaces of Comparative Examples 1 to 4. In the pure Ni-based alloy ((a), Comparative Example 1), an oxidized scale composed of two layers consisting of an outer layer containing Ni, Co and Cr as a primary component and an inner layer containing Cr and Al as a primary component was formed on the surface thereof, and an oxide was formed within the substrate. The oxidized scale has a thick thickness of 100 μm or more, and the peeling of the film was observed at many portions.

In the Ni-based alloy (b) subjected only the Al diffusion coating in Comparative Examples 2, while an oxidized scale has a thickness of about 15 μm and contained Ni, Cr, Al and Co as a primary component, the film had peeled portions as with Comparative Example 1. Further, while the Al concentration of the Ni—Al diffusion layer was about 50% before oxidation, it was reduced at about 25% after oxidation. Therefore, it can be proved that the reduction of the Al concentration was caused by the accelerated consumption of Al due to a high oxidizing speed, and the diffusion of Al to the substrate.

In the Ni-based alloy (c) subjected to the Al diffusion coating after coating the 9.2% Re-47.9% Ni-19.4 Cr-4.3% Co-16.5% Al-1.4% Ti alloy coating in Comparative Example 3, while the peeling level of the film was low as compared to Comparative Examples 1 and 2, an oxidized scale having a thickness of about 12 μm and containing Ti and Nb in addition to Al, Ni and Co as a primary component was created.

Given that the oxidized scale parabolically grows, if there is a 6 times difference in thickness between the oxidized scales of two different film, in the same time-period, these films will have a 36 times difference in life span therebetween. Thus, it can be said that the difference in thickness between the oxidized scales of the Inventive Example 1 and the Comparative Example 3 is significant.

Further, while the Al concentration of the Ni—Al diffusion layer was close to 50% before oxidation, it was reduced at about 27% after oxidation. The Al concentration in the vicinity of the surface of the substrate was also increased up to about 15%. Therefore, it can be proved that even if the 9.2% Re-47.9% Ni-19.4 Cr-4.3% Co-16.5% Al-1.4% Ti alloy coating having a low Re concentration is coated, Ti and Nb will be diffused from the substrate from the coating layer, and Al will be diffused from the Ni—Al diffusion layer to the substrate, during oxidation at 1100° C.

The Ni-based alloy (d) subjected to the Al diffusion coating after coating the 8.9% Re-7.9% Ni-56.2% Cr-3.9% Co-19.2% Al-2.0% Ti alloy coating in Comparative Example 4, exhibited a similar behavior to Comparative Example 3 to create an oxidized scale having a thickness of about 10 μm and containing Ti and Nb in addition to Al, Ni, Co and Cr as a primary component.

While the Al concentration of the Ni—Al diffusion layer was close to 50% before oxidation, it was reduced at about 27% after oxidation. Therefore, it can be proved that even if the 8.9% Re-7.9% Ni-56.2% Cr-3.9% Co-19.2% Al-2.0% Ti alloy coating having a low Re concentration is coated, Ti and Nb will be diffused from the substrate from the coating layer, and Al will be diffused from the Ni—Al diffusion layer to the substrate, during oxidation at 1100° C.

As seen in the above results, it is verified that Inventive Example 1 (38.1% Re-48.2% Cr-9.2% Ni alloy coating (atomic composition)) and Inventive example 2 (24.2% Re-39.5% Cr-31.8 Ni alloy coating (atomic composition)) as examples of the present invention bring out sufficient function as a diffusion barrier layer against oxidation at 1100° C. for one month to provide excellent oxidation resistance.

By contrast, in not only the pure Ni-based alloy, but also the Ni-based alloy subjected only to the Al diffusion coating, and the Ni-based alloy coated with the low-concentration Re alloy coating and the Al diffusion layer, a thick oxidized scale is formed even if the Al concentration of the surface of the coating layer is approximately the same as Inventive Examples 1 and 2, before oxidation. This is caused by the lowered Al concentration in connection with the diffusion of Al in the Al alloy layer to the substrate, and the deteriorated purity of Al₂O₃ in connection with the diffusion of Ti and/or Nb from the substrate to the oxidized scale, due to the activated diffusion between the respective elements of the substrate and the coating layer during the oxidation at 1100° C. for one month.

INDUSTRIAL APPLICABILITY

The diffusion-barrier ReCrNi alloy coating consisting essentially of a ternary alloy and, having, except for inevitable impurities, an atomic composition of 20% to 75% Re, an atomic composition of 20% to 60% Cr, and an atomic composition of 5% to 40% Ni can be coated on the surface of a high-temperature equipment member (or inserted between the substrate and the TBC layer) to provide excellent heat/corrosion resistances, and an extended life span as compared to conventional techniques. 

1. A coating for diffusion barrier, comprising: a first layer of an alloy containing Re formed on a substrate; and a second layer formed on said first layer having a composition of σ phase, comprising a ternary alloy except for inevitable impurities, in atomic composition, consisting essentially of: 24.2% to 38.1% of Re; 39.5% to 48.2% of Cr; and 9.2% to 31.8% of Ni, wherein said first layer functioning as a stress relief layer.
 2. The coating for diffusion barrier as defined in claim 1, further comprising a diffusion layer formed on said coating, said diffusion layer containing at least one selected from the group of elements consisting of Al, Si and Cr wherein said coating contains substantially none of said elements of said diffusion layer after a homogenizing heat treatment.
 3. The coating for diffusion barrier as defined in claim 1, wherein said substrate is made of a Ni-based alloy.
 4. The coating for diffusion barrier as defined in claim 1, wherein said ReCrNi alloy coating is formed as a continuous layer.
 5. The coating for diffusion barrier as defined in claim 4, further comprising a diffusion layer laminated on said second layer, said diffusion layer containing at least one selected from the group consisting of Al, Si and Cr.
 6. The coating for diffusion barrier as defined in claim 5, further comprising a heat barrier ceramics laminated on said coating. 